The present invention relates to a steering device for vehicle which changes the steering angle in response to the movement of an electrical actuator.
In a steering device for vehicle adopting a steer-by-wire system, movements of an electrical actuator for steering in response to operation of a steering wheel or other operation member are transmitted to the vehicle wheels such that the steering angle changes without mechanically linking the operation member to the vehicle wheels. It has been proposed that vehicles adopting such a steer-by-wire system calculate a target yaw rate or other target value corresponding to the vehicle behavior according to the amount of operation of an operation member, and execute control of the steering angle via the steering electrical actuator such that the target yaw rate or other target value coincides with the actual yaw rate or other actual value. It has also been proposed that, in order to prevent instability of vehicle behavior in cases where the friction coefficient between the road surface and vehicle wheels is reduced due to freezing of the road surface or other causes, the vehicle braking force is controlled such that the target yaw rate or other target value coincide with the actual yaw rate or other actual value. In addition, enhancement of the stability of vehicle behavior through integration of steering angle control and braking force control is also being studied.
It is required that the output of the steering electrical actuator overcomes the steering resistance arising from friction between the vehicle wheels and the road surface. In a steering device for vehicle adopting a steer-by-wire system in particular, if a steering force is to be generated solely by a steering electrical actuator, a large output is required. Consequently there is the problem that the steering electrical actuator must be large, so that actual application of a steer-by-wire system is difficult.
An object of this invention is to provide a steering device for vehicle which can resolve the above problem.
The steering device for vehicle of the present invention comprises a steering electrical actuator; means for transmitting movement of the steering electrical actuator to vehicle wheels such that steering angle changes according to the movement; a pump to supply hydraulic fluid; a brake device; a steering assistance force generation hydraulic actuator; means for judging whether the vehicle is steered in a stationary state; and, means for switching the supply route of the hydraulic fluid in response to the judgment result such that, the hydraulic fluid can be supplied from the pump to the steering assistance force generation hydraulic actuator when the vehicle is steered in the stationary state, and the hydraulic fluid can be supplied from the pump to the brake device when the vehicle is not steered in the stationary state.
During steering at zero vehicle speed, that is, when the vehicle is steered in the stationary state, there is no need to amplify or control the braking force of the brake device. Therefore, when the steering angle is steered by means of the steering electrical actuator in the stationary state, the hydraulic pressure for braking can be diverted to assist the steering. Thus by means of the arrangement of the present invention, the steering electrical actuator can be made small and lightweight, and the energy efficiency of the vehicle can be improved.
It is preferable that the movement of the steering electrical actuator is transmitted to the vehicle wheels without mechanical linkage of the operation member with the vehicle wheels. By this means, the utility of a vehicle adopting a steer-by-wire system can be improved.
It is preferable that the means for transmitting the movement of the steering electrical actuator to the vehicle wheels is comprised of a steering gear; that the means for switching the supply route of the hydraulic fluid is comprised of an electromagnetic switching valve; and that the means to judging whether the vehicle is steered in the stationary state is comprised of a computer.
According to the present invention, the hydraulic pressure for braking is diverted to assist steering when the steering angle is changed by means of the electrical actuator in the stationary state, and thus a steering device for vehicle can be provided in which the electrical actuator can be made smaller and lightweight without complicating the constitution, the energy efficiency of the vehicle can be improved, and the utility of a vehicle adopting a steer-by-wire system can be enhanced.